For machining work-pieces, by cutting, turning, milling, drilling and like, cutting tools are used. The cutting tools remove surplus material, henceforth chips, thereby shaping the work-piece. However, they are, themselves, worn away in the process and require replacing. There is a correlation between hardness and wear resistance. To ensure that chips are efficiently removed from the work-piece, whilst ensuring long working life of the cutting tool, the cutting tool is required to be hard and tough.
Hardness however may be correlated with brittleness however. Being both hard and tough, composite materials consisting of hard ceramic particles in a metal matrix are very popular choices for cutting tools. A number of such ceramic-metal composites or cements have been developed. The so-called hard metals consisting of tungsten carbide particles in a metal matrix such as cobalt for example, are the materials of choice for fabrication of cutting members for many applications. The term “cutting member” includes, for example, inserts, cartridges, cutting plates, solid carbides cutting heads, drills and end mills, etc.
The term “wear part” describes components used in applications where wear is a recognized problem. Wear parts may be for various wear applications such as, for example, machine parts, textile machine parts, ball bearings, roller bearings, moving parts in heat exchangers, turbo loaders, gas-turbine, exhaust valves, nozzles, manufacturing process dies for example for extrusion or wire drawing, punches, blanking tools, hot forging and pressing, molds, shear blades, plunger rods for pumps, plunger ball blanks, down hole pump check valve blanks, bushings, and other wear and impact applications.
Wear parts are commonly made of carbon steel, austenitic, ferrific or martensitic stainless steels, hot work tool steels, cold work tool steels, 51000 steels, nickel and cobalt super alloys, and high speed steel.
It will be appreciated that the wear of cutting members and wear parts takes place at their contact surfaces, and can be attributed to mechanical or friction type wear or abrasion. Abrasion of cutting tools is often enhanced by chemical attack, such as oxidization, for example, where the cutting tool material reacts with the surrounding air, and/or the work-piece and/or coolant fluids and lubricants in wet machining processes.
The downtime of cutting tools whilst the cutting members are replaced and of other applications in which wear parts are replaced is costly. Much research is directed to improving the wear resistance of such cutting tools and wear parts by application of hard and/or chemical resistant coatings to increase their working life.
Indentation hardness is a measure of resistance to plastic deformation. There is a strong correlation between indentation hardness and Mohs hardness, which indicates the relative resistance of materials to scratching. In general, the harder a material, the more abrasion resistant it is.
Since hardness is a measure of resistance to plastic deformation, unfortunately, there is a general correlation between hardness and brittleness, and the harder a material is, the more brittle it is, i.e. the more likely it is that stresses will be relieved by crack propagation instead of by plastic deformation. In consequence of the above, it is generally found that the more resistant a material is to gradual abrasion, the more it is likely to be susceptible to brittle failure. It is often found that coatings that resist slow wear tend to be susceptible to catastrophic failure modes such as thermal shock, spalling, coating delamination and the like.
The general thrust of materials science research and surface engineering for cutting tools and wear parts is to develop hard, tough (non-brittle) coatings that increase the working life of cutting tools and wear parts by providing protection on the surface against the main causes of wear: heat, chemical attack and abrasion.
Coatings may be formed on cutting members and wear parts by a range of coating technologies that are generally classified as PVD (physical vapor deposition) or CVD (chemical vapor deposition).
PVD gives very good properties and coating deposition rates are generally equivalent than those of CVD techniques. It is a feature of PVD processes that coatings can only be applied to line-of-sight areas of a substrate and cannot be applied in holes and on shielded surfaces. Residual stresses from coating deposition tend to be compressive and these stresses may cause coatings to flake off. Because of both the low deposition rates and the risk of coating failure due to the tensile internal stresses and residual stresses from the deposition process as the coating thickness increases, PVD is generally limited to thin coatings.
In contrast, CVD coatings are not restricted to line-of-sight deposition. Relatively thick coatings of several microns may be deposited and, since residual stresses may be tensile or compressive depending upon the substrate, the coatings are less susceptible to spalling. Furthermore, deposition temperatures are typically rather higher than those of PVD technologies. This facilitates the development of a diffusion-induced interface between the coating and substrate which allows good adhesion to be achieved. Indeed, good adhesion is one of the critical requirements for the coatings applied to cutting members and wear parts and for more than 40 years, CVD (chemical vapor deposition) has been used for coating cutting tools, cutting members, and wear parts thereby improving their performance and effective working life.
It will be appreciated that some coatings and coating—substrate combinations favor themselves to one or other deposition process and there are host of materials for which only one or other process route is practicable.
Coatings of TiN, TiC and Ti(C,N) may be deposited onto appropriate substrates by reacting titanium tetrachloride with other gases, and removing the gaseous chlorides thus formed:TiCl4+N2+H2→TiN+Chlorides and other gases.TiCl4+CH4+H2→TiC+Chlorides and other gases.TiCl4+N2+CH4+H2→Ti(C,N)+Chlorides and other gases.
It will be appreciated that, over the years, other chemical vapor deposition routes have become available for deposition of TiN, TiC and Ti(C,N), and the titanium chloride processes described above are given by way of non-limiting example, only.
For example, MT (medium temperature) processing routes which tend to produce different microstructures, often having columnar grain structure are popular. For example:CH3CN+N2+H2+TiCl4→MT Ti(C,N)+Chlorides and other gases.
With reference to FIG. 1, a scanning electron micrograph of the face of a typical MT-Ti(C,N) coating as deposited by CVD is shown. The coating typically presents a fine grained (1-3 micron) grain size on its face.
As the processing temperature increases, the substrate expands. On cooling, the substrate and coating contract and, if the contraction is at different rates, residual stresses result. It will be noted that the crack to the right of the micrograph is a typical consequence of thermal mismatch between the coating and the substrate. By lowering the process temperature, such cracking can be minimized. Where substrates contract more than coating on cooling, such cracks tend to be closed.
Examination of a section through such a coating shows that the microstructure consists of elongated crystals aligned through the coating thickness. This is due to the growth of seeded crystals aligned such that the preferred direction of growth lies through the coating thickness. Such coatings may be as much as 30 microns thick.
Cemented carbide made primarily of tungsten carbide optionally with the addition of other carbides in a primarily cobalt binder is by far the most popular substrate used for cutting tools. To prevent the cobalt binder reacting with the CVD gases used for depositing a wear resistant coating such as TiCN, a thin (0.1 μm to 1.5 μm) protective layer of TiN is generally deposited prior to the TiCN layer. The protective layer of TiN allows the tool bit to be subjected to the relatively harsh CVD conditions required for deposition of TiCN without decarburizing the substrate thereof, thereby minimizing the formation of undesirable, brittle η phases (M12C, M6C where M is Co and W) being formed near surface of the substrate 12. EP 0 440 157 and EP 0 643 152 describe deposition of TiN under TiCN in this manner.
TiCN is preferred to TiN in many cutting tool applications since TiCN has better wear resistance and a lower coefficient of friction than TiN. Indeed, machining with a TiN surfaced cutting tool may result in very high temperatures being generated at which the coating may oxidize.
In the machining of hard materials, such as cast iron, for example, high temperatures are generated and even TiCN, and TiC may interact with the work-piece and/or with the cooling fluids and air.
One way of limiting workpiece—coating reactions is by alloying the coatings with silicon which tends to form dense oxides. Alloying with chromium or vanadium increases toughness and thus tool life when machining certain applications.
U.S. Pat. No. 6,007,909 to Rolander et al., entitled “CVD-Coated Titanium Based Carbonitride Cutting Tool Insert” relates to a cutting tool insert of a carbonitride alloy with titanium as the main component but also containing tungsten and cobalt. The cutting tool insert is useful for machining, specifically for the milling and drilling of metal and alloys. The insert is provided with a coating of at least one wear resistant layer. The composition of the insert and the coating is chosen in such a way that a crack-free coating in a moderate (up to 1000 MPa) compressive residual stress state is obtained. It is alleged that the absence of cooling cracks in the coating, such as that shown to the right of FIG. 2 and described hereinabove, in combination with the moderate compressive stress, gives the tool insert improved properties compared to prior art tools in many cutting tool applications. The alloying of the coatings with Ti, Al, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Si or B to give solid solutions is discussed. The coatings are characterized as being free from cooling cracks, having a thickness exceeding 1 μm and a compressive residual stress at room temperature of 100-800 MPa. It will, however, be appreciated that using titanium based carbonitride as the substrate for machine tool inserts is a serious limitation. For regular cutting tools, WC—Co is the material of choice. Furthermore, although V, Cr and Si are suggested as possible alloying elements for addition to coating layers during CVD deposition, there is no further discussion of such coatings, and it does not appear that they were ever produced.
There is thus still a need for improved Ti based hard metal coatings and the present invention addresses this need.